FIG. 1 shows a partial cross-section of a known MRI (magnetic-resonance imaging) system 10 in operation. A field magnet 12 and gradient coils 14 with edge sections 17 provide a homogeneous magnetic field in an imaging region 15. RF coils 16 generate high energy RF fields, as required for MRI operation. A cover 18 covers the RF coils 16, gradient coils 14 and the field magnet 12. A patient bed 19 may also be provided, which provides support for a patient 20. The cover 18 is contactable by the patient. They patient 20 may contact the cover, for example with arms left to dangle beside the bed. A gap 22 typically exists between the RF coils 16 and the cover 18. The MRI system may be in the form of a cylinder, with the patient placed in the middle of the cylinder. The cover 18 may then form a cylindrical inner surface, for receiving the patient. In such instances, the cover may be referred to as a patient bore liner. The gas pressure in gap 22 may be reduced to 1-100 mbar, in order to reduce the transfer of acoustic noise from the magnet coils 12, 14, 16 to the patient 20.
This invention is concerned with breakdown between any high voltage, high frequency source and a human or animal body in proximity to the high voltage, high frequency source in a partial vacuum. The partial vacuum is present to reduce acoustic noise transfer, but has the unwanted side effect of reducing the breakdown voltage from the magnet coils to the cover.
FIG. 2 shows a partial cross-section of the system of FIG. 1, taken along the line II-II. The electrical properties of the structure of FIG. 2 will now be considered. Due to the nature of the human body, the patient 20 may be considered to be an electrical earth. RF coils 16 produce high voltage, high frequency signals across the gap 22 and the cover 18 to earth at the patient 20.
Simulations performed on one of the applicant's existing products—which includes a high frequency high voltage RF coil—showed RF electrical breakdown between the gradient coil 14 and the inside of the cover 18. The simulation was based on the arrangement shown in FIG. 1. From this simulation, a capacitive model was derived to describe operation of the system in breakdown conditions.
It is important to prevent any voltage breakdown across the gap 22, since such breakdown may cause electrical arcing of high voltage, high frequency RF energy from the RF coils 16 to the patient 20. In some MRI systems, the RF power may attain values in excess of 15 kW. Arcing of such power levels to the patient would cause injury. Considering the voltage distribution in steady state, the high voltage high frequency voltage VRF is split between a voltage drop Vg across the gap 22 and a voltage drop Vs across the material of the cover 18. The patient 20 is conductive and at an assumed ground voltage, so that VRF=Vg+Vs. The relative values of Vs and Vg are determined according to the ratio of the inverse of the respective capacitances Cs, Cg of the corresponding layers. In certain known arrangements, it has been found that 70% of the RF voltage is supported by the gap 22, while 30% is supported by the material of the cover.
FIG. 4 shows a schematic diagram representing the capacitive model as used in the simulations mentioned above. In FIG. 4, C1 represents the capacitance between the RF coils 16; C2 and C3 represent the capacitance of the gap 22 between the gradient coil 14 and the cover 18, at respective ends of the apparatus; C4 represents the surface capacitance of the surface of the cover 18 which is exposed to the gap 22; C6 and C7 represent the capacitance of the material of the cover 18, at respective ends of the apparatus, and C5 represents the surface capacitance of the surface of the cover 18 which is directed toward the patient 20.
When no patient is in contact with the cover 18, the model assumes symmetry of the equipment, such that C2=C3, C4=C5 and C6=C7. With a patient in contact with the cover 18, this symmetry no longer holds. The capacitance C5 falls to zero, since the patient provides a resistive path in good electrical contact with the surface of the cover 18. The model in this state is shown in FIG. 5. With a patient in place, the capacitance C2, C3 of the gap 22 and the capacitance C6, C7 of the material of the cover 18 become dominant in defining the voltage Vg across the gap 22 (FIG. 2). In this situation, the proportion Vg of the total voltage VRF which is borne across the gap 22 is proportional to the distance dg across the gap. This follows from the fact that the capacitance Cg across the gap decreases in proportion to an increase in gap size dg. In order to reduce the likelihood of voltage breakdown between the high voltage, high frequency source (RF coils 16) and the patient, the proportion Vg of the total voltage VRF which is borne across the gap must be reduced, and/or the proportion Vs of the total voltage VRF which is borne across the cover must be increased.
Gap Size
One way of reducing the voltage Vg across the gap and so reduce the likelihood of voltage breakdown across the gap is to increase the capacitance across the gap by reducing the distance dg across the gap.
Permittivity of the Material of the Cover
The voltage Vg across the gap 22, being equal to (VRF−Vs), may be seen to be vary inversely with ds, the thickness of the cover 18, and proportional to εrs, the relative permittivity of the material of the cover 18. This follows from the fact that the capacitance Cg across the gap increases in proportion to εrs; while the capacitance Cs across the cover decreases in proportion to an increase in thickness ds. In an embodiment, εrs may be about 4.
Accordingly, the voltage Vg across the gap 22, which should be minimised, follows the following relationship:Vg∝εrs.dg/ds.
To achieve minimum Vg, one must minimise εrs, the relative permittivity of the material of the cover 18, while reducing the thickness dg of the gap and/or increasing the thickness ds of the material of the cover 18.
Gas Pressure & Gap Thickness—Paschen's Law
Another aspect affecting the electrical breakdown voltage of a medium is the electrical strength of the medium. Paschen's law states that the breakdown voltage for a discharge between electrodes in gas is a function of the product (pd) of the pressure of the gas, and the distance between the electrodes. The function has a minimum value for air at pd=0.55 Torr cm (0.733 Nm−1), indicating a breakdown voltage 352 V between electrodes 0.01 m apart. In one expression of the Paschen equation, the breakdown voltage Vbg of a gap in air is given by:Vbg=6.72√(pd)+24.36(pd)kV.
Since we wish to increase the breakdown voltage across the gap, thereby to reduce the likelihood of voltage breakdown across the gap, operation at the Paschen minimum value must be avoided. Operation with a pressure distance (pd) product above or below that of the Paschen minimum will provide an improved breakdown resistance.
In known MRI systems, the air or other gas in the gap 22 is typically at a very low pressure but above the Paschen minimum. This will reduce the breakdown voltage of the gap 22 from that of an equivalent gap filled with air at atmospheric pressure. Since the gap usually contains gas at less than atmospheric pressure, there is a risk of electrical breakdown. It would be advantageous to increase the pressure of the gas in the gap, since the present invention aims to reduce the risk of electrical breakdown across the gap 22. However, this is not a practical proposition since acoustic noise transfer through the gap would increase.
For an example of the system considered by the present invention, a gap of 0.005 m in air at a pressure of 75 Torr (10000 Nm−2), a pressure-distance product pd of 37.5 Torr cm (50 Nm−1), gives a breakdown voltage of approximately 2.8 kV. This is well above the Paschen minimum. The same gap filled with air at atmospheric pressure, giving a pressure-distance product pd of 507 Nm−1, would have a breakdown voltage of about 16.9 kV; a marked improvement over the breakdown voltage for the low-pressure gap.
On the other hand, if the pressure of the gas in the gap is reduced, less acoustic noise would be transmitted to the patient 20 from the MRI system 10. The reduced pressure gas is less efficient as a transport medium for sound. Since the patient 20 may be enclosed in a relatively small cylindrical enclosure, it is important to reduce the acoustic noise heard by the patient, since high levels of acoustic noise could increase a patient's feeling of stress when contained in the cylindrical enclosure. However, a reduced pressure of the gas in the gap 22 would lead to a lower breakdown voltage across the gap.
There are accordingly conflicting requirements for the pressure of the gas in gap 22. A reduced pressure is desirable for reduction of acoustic noise transmission, while an increased pressure is desired for increasing the breakdown voltage of the gap.
Reactance of Material of the Cover
An electrical breakdown of the gap 22 would in effect provide a resistive path between the high voltage, high frequency source (RF coils 16) and the patient 20. This could be harmful to the patient. By restricting the connection between the high voltage, high frequency source (RF coils 16) and the patient to a capacitive one, this risk is eliminated. The present invention aims to reduce the possibility of a resistive path being established between the high voltage, high frequency source (RF coils 16) and the patient 20.
To further reduce the proportion Vg of the total voltage VRF across the gap 22, the proportion Vs held across the material of the cover 18 may be increased. This may be achieved by providing a material of the cover 18 of higher reactance X.